1. Field of the Invention
The present invention relates to a ceramic substrate for thin-film electronic components, to a method for producing the ceramic substrate, and to a thin-film electronic component employing the substrate. More particularly, the invention relates to a ceramic substrate having excellent surface smoothness for use in thin-film electronic components, to a method for producing the ceramic substrate, and to a thin-film electronic component employing the substrate.
2. Description of the Related Art
In recent years, there has been a keen demand for thin-film electronic components making use of a thin film member; e.g., a thin-film capacitor having small dimensions and large capacitance. These thin-film electronic components; for example, thin-film capacitors, are required to contain a conductor layer and a dielectric layer of as small a thickness as possible. Therefore, these layers are generally formed through a thin-film formation technique such as a sputtering method, a CVD method, or a sol-gel method. During formation of these thin layers, surface conditions of a substrate on which thin film is to be formed are key factors. When the substrate has a surface of poor flatness, desired characteristics cannot be reliably attained, and various other problems also arise, such as insufficient interlayer insulation. In this connection, there have been known substrates for providing a surface of high flatness, and the below-described Patent Documents 1 and 2 disclose a glazed ceramic substrate which is produced by coating a ceramic substrate with a glass material.
Patent Document 1    Japanese Patent Application Laid-Open (kokai) 2001-044073
Patent Document 2    Japanese Patent Application Laid-Open (kokai) 2003-017301
3. Problems to be Solved by the Invention
The aforementioned Patent Document 1 discloses that a planarization film such as a glass coating may be deposited in order to enhance flatness. The above Patent Document 2 discloses that a glazed alumina substrate successfully provides a flat surface. However, as described in Patent Document 2, even when a conventional glazed ceramic substrate formed of high-purity (99.5% or higher) alumina is employed, the arithmetical mean roughness (Ra) of the surface of the glazed ceramic substrate is at best about 30 nm. In consideration of the requirements for thin-film electronic components in recent years, a surface of higher flatness is in demand, but attainment thereof through a conventional technique has been difficult.